Ligand chelating bisphosphines

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

Arylation of alkynes via addition of arylboronic acids to alkynes represents an attractive strategy in organic synthesis. The first addition of arylboronic acids to alkynes in aqueous media catalyzed by rhodium was reported by Hayashi et al.89 They found that rhodium catalysts associated with chelating bisphosphine ligands, such as 1,4-Ws(diphenyl-phosphino)butane (dppb) and 1,1 -/ E(diphenylphospliino)fcrroccnc... 

The weak Si-Pgq interactions in 9 should be easily broken, giving a chelating bisphosphine ligand to transition metals. With the "phosphophilic" Ni(0) metal center, one diphosphinomethanide ligand is transferred completely. 

Even though the outlined approach allowed the successful rationalisation of many experimentally observed shift/structure and shift/reactivity correlations, Leitner et al. have pointed out that such relations cannot be expected to be universally valid and require that structural variations are modest and avoid large simultaneous changes in parameters that may have opposite effects on metal chemical shifts.61 To overcome these drawbacks and establish a more rational interpretation of chemical shift trends, they used a combination of experimental and computational efforts to assess the importance of different electronic and structural factors on the metal chemical shifts of a series of rhodium complexes with bidentate chelating bisphosphine ligands. The basis of their approach is first the validation of experimentally observed metal shifts by... 

The chiral phosphine 31 or 32-rhodium complex catalyzed the addition of arystannanes 30 to N-sulfonylimines 29 to give diarylmethylamines 33 with high enantioselectivity (75-96% ee) [21]. The choice of the chiral monoden-tate phosphine ligand is essential for their catalytic asymmetric arylation. With chelating bisphosphine ligands the arylation was very slow. The authors hypoth-... 

Nickel catalyst complexed with unfunctionalized chelating bisphosphine ligands, (R,R) norphos (75) 151 ] and 76 [ 19,52], also induced a high selectivity in the reaction shown in Scheme 8F.5 (Table 8F. 1, entries 38-39). The results reported with other phosphine ligands 33, 77-80 [30,53-56] are summarized in the Table 8F.1 (entries 40-44). 

P-31 NMR Studies of Equilibria and Ligand Exchange in Triphenylphosphine Rhodium Complex and Related Chelated Bisphosphine Rhodium Complex Hydroformylation Catalyst Systems... 

P-31 NMR studies also were carried out in a similar manner on rhodium complexes of two chelating bisphosphines—bis-l,3-diphenyl-phosphinopropane and bis-l,2-diphenylphosphinoethane. These complexes were generated in solution via ligand displacement from tris(tri-phenylphosphine)rhodium carbonyl hydride. For example, one of the possible displacement products of bis-l,3-diphenylphosphinopropane (F) is a cis-chelate (G) that can undergo dissociation to yield a chelating bisphosphine complex (H) ... 

Knowles [1] and Homer [2] independently discovered homogeneous asymmetric catalysts based on rhodium complexes bearing a chiral monodentate tertiary phosphine. Continued efforts in this field have produced hundreds of asymmetric catalysts with a plethora of chiral ligands [7], dominated by chelating bisphosphines, that are highly active and enantioselective. These catalysts are beginning to rival biocatalysis in organic synthesis. The evolution of these catalysts has been chronicled in several reviews [8 13]. 

This section is dedicated to first-sphere coordination calixarenes connected to a metal for which another ligand might temporarily penetrate in the host cavity. For that purpose, the idea of rational catalyst design could be taken a step further by using sterically hindered chelated bisphosphines or bisphosphites with a calix[4]arene backbone. 

Although the Heck reaction mainly involves the formation of bonds between sp -hybridized carbon atoms, there are asymmetric versions. One approach is to capture the chirality of the insertion intermediate by ensuring the p-hydride elimination either occurs away from the site of the original alkene (Scheme 5.35), or is pre-empted by a different step such as another insertion in a tandem process. Another approach is to provide a symmetrical substrate with two enantiotopic alkenes (Scheme 5.36), an approach also used in metathesis chemistry (Section 8.3.6). In all of these reactions, the source of chirality is from the employment of chiral ligands for the palladium catalyst, often chelating bisphosphines. 

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